(i) Field of the Invention
This invention relates to a novel type of lubricating oil compositions and more particularly, to lubricating oil compositions which comprise esters constituted with glycerol, fatty acid and boric acid having specific ratios of carboxylic acid residue, glycerol residue, and boric acid residue.
(ii) Description of the Prior Art
In recent years, there is an increasing tendency toward the saving of energy. The energy saving such as in automobiles or ships has become one of great concerns. For the purpose, improvements have been steadily made on so-called hardwares such as materials and structures.
On the other hand, attention has been paid to a frictional energy loss in lubrication. Extensive studies have been made on improvements of lubricating oils in order to reduce the loss and thus to decrease energy or fuel consumption.
One of methods of reducing the frictional energy loss is to add friction reducing additives to lubricating oils.
For instance, Japanese Laid-open Application (Japan kokai) No. 55-66996 discloses fatty acid amides of diethanolamine or mixtures thereof with fatty acid esters of diethanolamine as friction reducing additive. In Japanese Laid-open Application (Japan kokai) No. 55-84394, there is described an improvement of fuel economy of internal combusion engines by addition of fatty acid glycerol esters to lubricating oils. Although these additives added to lubricating oils are found to show an improvement of fuel economy over lubricating oils containing friction reducing additives, most of them have a difficulty in practical applications because they allow a degree of corrosion in bearings at the inside of an engine mechanism to become greater than the tolerance limit of corrosion.
Japanese Laid-open Application (Japan kokai) No. 56-141398 discloses sorbitan boric esters as friction reducing additive. It is described that lubricating oil compositions comprising an effective amount of the esters can not only reduce the consumption of fuel, but also reduce a degree of corrosion at bearing portions to a minimum. However, when sorbitan boric esters are allowed to stand in air, they tend to form a thick solid film deposit on portions where exposed. Once formed, the solid deposit does no longer dissolve in lubricating oils.
As for other boric esters, boric esters of glycerol fatty acid monoesters are known in this field and are referred to in U.S. Pat. No. 2,795,548 as a corrosion inhibitor being incorporated in lubricating oils, in U.S. Pat. No. 3,117,089 as an anti-rust agent being incorporated in fuels and lubricating oils, and in Japanese Laid-open Application (Japan kokai) No. 56-150097 (European Patent Application No. 0036708) as friction reducing additives being incorporated in lubricating oils. However, in such incorporation of boric esters of glycerol fatty acid monoesters in lubricating oils, when the proportion of boric acid to glycerol fatty acid monoester is small, a degree of corrosion in bearings is allowed to become greater than the tolerance limit, on the other hand, the greater the proportion is set, the more they tend to form a thick solid film under the conditions of air-contact. Thus, it is difficult to obtain a lubricating oil which can satisfy both non-corrosive and friction reducing properties at the same time.